1. Field of the Invention
A control system for an automatic automotive transmission typically includes various frictional elements, such as clutches and brakes, which are controlled by a hydraulic circuit so as to form various paths of power transmission, thereby placing the automatic transmission in any desired range, such as a reverse range and several speed ranges in a forward range.
2. Description of Related Art
Such an automatic transmission control system includes a hydraulic circuit provided with a bypass pressure line with a control valve that allows oil to flow so as to bypass a throttle element, such as an orifice, in a pressure line for the frictional elements that are locked when the transmission is placed in a low speed range or in a middle speed range. The control valve is operated to open the bypass pressure line upon a gear shift from a neutral range to drive range when the temperature of oil is low, or upon an automatic gear shift during high speed cruising. Such an automatic transmission control system is known from, for instance, Japanese Unexamined Patent Publication No. 62- 83535.
In the automatic transmission control system, if the temperature of oil is low upon shifting the transmission into the drive range, the hydraulic oil is introduced to a frictional element through the bypass pressure line in order to avoid a delay in locking action of the frictional element due to an increased oil viscosity. If the automatic transmission control system has a frictional element that is locked and unlocked during running and is provided with what is known in the art as a "drift-on ball", the bypass pressure line is opened to introduce the hydraulic oil into the frictional element in order to cause with certainty the closing of the drift-on ball when the frictional element is locked during a high speed cruising.
The drawback to the use of such a prior art automatic transmission control system is that the frictional element requires a longer time to achieve its locking for placing the transmission in a reverse range or in a drive range from a neutral range, particularly when gear shifting is made after acceleration, or when acceleration and gear shifting are simultaneously performed, and generates an increased amount of heat due to slippage. In more detail, a frictional element that is locked to place the transmission in a desired range is, generally, supplied with hydraulic oil through a throttle element, such as an orifice, in order to reduce locking impact. Nevertheless, when an engine load is increased due to an acceleration during the shifting of the transmission into the desired range, the frictional element increases torque, so as to take a longer time for completing the operation of locking. During the locking operation, the frictional element allows some slippage. This results in an increase in temperature of the frictional element.